Hydraulic pump

ABSTRACT

A piston type hydraulic pump characterized by a relief clutch apparatus which functions to detect the occurrence of contaminates in the pumping oil and to automatically protect the internal working components of the pumping mechanism from damage by such contaminates. More specifically, the relief clutch is arranged to automatically and substantially instantly relieve the pumping pistons from the forces of the piston driving cam upon the occurrence of any abnormal forces imposed on any one of the pistons by binding action of the contaminates. Hence actuation of the pistons is automatically stopped prior to the occurrence of irreparable damage to the machine.

United States Patent 1191 Raymond Aug. 27, 19 74 HYDRAULIC PUMP v Primary ExaminerWilliam L. Freeh [75] lnvemor' 622:" Raymond Zanesvlue Assistant Examiner-Gregory Paul LaPointe Attorney, Agent, or Firm-Palmer Fultz, Esquire [73] Assignee: International Basic Economy 1 Filed: 73 A piston type hydraulic pump characterized by a relief Pi l. No.: 344,339 clutch: apparatus which functions to detect the occur- 1 rence of contaminates in the pumping oil vand'to auto- 1 matically protect the internal working componentsof [52] US. Cl 417/222, 417/223, 417/270 1 the pumping mechanism f damage by such [51] Int. Cl. F04b 1/00 taminates More ifi ll the li f clutch is an [58] F leld of Search 417/214, 223, 269, 270, ranged to automatically and substantially instantly 417/222; 192/58, 56 R lieve the pumping pistons from the forces of the piston V driving cam upon the occurrence of anyabnormal References u v forces imposed on any one of the pistons by; binding UNITED STATES PATENTS action of the contaminates. Hence actuation of the 2,953,099 9/1960 Budzich 417/269 Pistons is automatically pp Prior o. the 00611!- 2,962,970 12/1960 Norlin 417/270 ence of irreparable damage to the machine. ,160,110 12/1964 Budzich 417/2'70 1 1 1357,36; R 12/1967 Ray1n ond.'....... ..r..'417/270 8 s WWW Figures 1 1 71021 38 5/ 1 145 87 5a 44 i t 146 4 56 l/O-A 1 2 A 32 93 I ,2 22

I 5?; S o v 1 1L1 /oa 3 t 2& 300 302 /42 fi I 0 94 4 /\90 //1 /I\ 9 I28 Corporation, New York, NY.

' ABSTRACT PATENTEU 3582.094

SHEET 2 i i 3 HYDRAULIC PUMP BACKGROUND OF THE INVENTION The present invention relates generally to hydraulic pumping apparatus and more particularly to axial piston type pumps.

PROBLEMS IN THE ART In the operation of pumps of hydraulic systems the problem of damage by contaminates to internal components of the pump has long been troublesome. More particularly, damage to the pumping pistons and cylinders by metal chips or other foreign materials in the oil usually starts out gradually, in a heretofore undetectable manner, with progressively increasing wear and galling of the metal parts to the point where irreparable damage to the pistons and cylinders has occurred, or indeed to the point of catastrophic failure of the entire pumping mechanism.

In accordance with the present invention a novel relief clutchapparatus is provided which not only detects at the very beginning the occurrence of damaging forces by contaminates, or excessive temperatures or pressures, but also functions to automatically and instantaneously relieve the pumping pistons and cylinders from the driving force of the piston actuating cam. Hence immediate protection is afforded the pump prior to the occurrence of damage to the internal components.

SUMMARY OF THE INVENTION In general the hydraulic pump of the present invention is provided with a novel relief clutch apparatus that functions to automatically relieve the pumping pistons from the force of the piston driving cam in the event that piston movement is impeded or obstructed by the presence of contaminates in the oil being pumped. The novel relief clutch apparatus is uniquely adapted to relieve the pistons of the cam force substantially instantaneously, within a portion of a piston stroke, and thereby prevent damage to the pistons, cylinders, as well as other internal components of the pumping apparatus.

The relief clutch apparatus is further adapted to selectively be made operatively responsive to the occurrence of unusual and damaging temperatures or system pressures thereby providing important additional pump and system protection cap abilities.

It is, therefore, a primary object of the present invention to provide a novel hydraulic pump that includes a unique relief clutch apparatus which functions to automatically protect the internal working components of the pump from damage by contaminates such as metal chips or other foreign matter in the pumping oil.

It is another object of the present invention to provide a novel relief clutch apparatus for a piston type pump which apparatus automatically functions to protect the internal working components of the pump from damage by excessive temperatures or pressures in the event such should occur in the hydraulic system.

While the forms of embodiments of the present invention as herein disclosed constitute preferred forms, it is to be understood that other forms might be adopted.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. I is a side elevational view of a pumping apparatus constructed in accordance with the present invention, the section being taken along a vertical plane through the centerline of the apparatus;

FIG. 2 is a side sectional view of a relief clutch apparatus comprising a portion of the pump of FIG. 1;

FIG. 3 is a second side sectional view of the relief clutch apparatus of FIG. 2; I

FIG. 4 is a force diagram showing the forces to which the relief clutch apparatus, FIGS. 2 and 3, is subjected; and a FIG. 5 is a side sectional view of a modified relief clutch apparatus for use in the pump of FIG. 1 and comprising a modified embodiment of the present invention.

DESCRIPTION OF THE PREFERRED. EMBODIMENTS Referring in detail to the drawings, a variable displacement pump constructed in accordance with the present invention is illustrated in FIG. 1 and comprises a housing indicated generally at 20 that includes a front housing portion indicated generally at '22 and a rear housing portion indicated generally at 24. The housing portions are joined together at the central portion of the pump and held by a plurality of studs 26.

A drive shaft 28 is mounted in the forward end of the housing by tapered roller bearing assemblies 30 and 32 which are pressed .into recesses 34 and 36. I

An oil seal 38 is pressed into a recess 40 in housing 20 and includes an annular resilient element 42 that wipes the periphery of drive shaft 28.

As seen in FIG. 1, the inner end of drive shaft 28 carries a cam means indicated generally at44 which includes a central bore 46 provided with a keyway 48 that receives a key 50 for preventing rotationof cam means 44 relative to shaft 28. The cam means isretained on shaft 28 by a nut 52 which is tightened into locked relationship on a threaded inner end of shaft 28.

With continued reference to FIG. 1, cam means 44 includes an inclined surface 54 which engages a plurality of shoes 56 preferably formed of low friction material, the latter including sockets 58 whichformpivotal ball joints with ball-shaped ends 60 formed on a plurality of pumping pistons indicated generally at 62.

Each of the shoes 56 is surrounded by a metal casing that is crimped around the ball-shaped end of its respective piston 62. Details of the piston shoe construction are shown and described in detail in my US. Pat. No. 3,357,363 issued Dec. 12, 1967.

With continued reference to FIG. 1 a cylinder barrel cartridge assembly indicated generally at 70 is axially slideably mounted within housing-20 on longitudinally extending rods or barrel bearing members, not illustrated, but shown and described in detail in my above mentioned US. Pat. No. 3,357,363.

Pistons 62 are disposed in respective barrel cylinders 78 which receive low pressure oil or hydraulic fluid in a novel manner via housing inlet port 80, passage 84 in front housing portion 22, an inlet chamber indicated generally at 86 within housing 20 and inlet ports 88 formed in each piston 62.

Now referring to FIG. 1 eachcylinder 78 includes a unique outlet port construction whereby a first cylinder outlet port 81 is disposed in a side wall of each cylinder 78 and a second cylinder outlet port 83 is axially spaced from outlet port 81 and formed in the rearward end of cylinders 78.

As seen in FIG. 1 each of the cylinders 78 includes a respective reaction plug, indicated generally at 90 in free self-aligning engagement with the inner end surface 92 ,of rear housing portion 24.

Each reaction plug 90 is provided with a central bore 94 that carries an outlet check valve 96 which is freely retained in bore 94 by a threaded plug 98.

Each threaded plug 98 includes a seat portion 100, a longitudinal passage 102, and a radial passage 103, the latter communicating with an annular passage 104 formed in the outer wall of reaction plug 90.

With continued reference to FIG. 1, bore 94 in each reaction plug 90 includes a valve stop 106 and a compression spring 108 which serve to limit the stroke of the balland bias it toward a closed position.

A plurality of valve means 87, attached to the outer surface of barrel 70 by screws 89, covers each of the first cylinder outlets 81 and is in the form of a substantially flat, flexible reed-type valve. Valve means 87 is normally biased in a closed position and the degrees of valve openingis limited by a stiff bacleup plate 91 attached by screws 89 to valve means 87.

It is important to point out that the reed-type valves 87 are preferred because they provide efficient valve means and yet conveniently take up little space. Further, valves 87 are easily incorporated into the cylinder barrel cartridge assembly 70 to provide an integral package to offer the advantages described in detail in my above mentioned U.S. Pat.'No. 3,357,363.

Pressurized fluid from cylinders 78 is first discharged on the compression stroke of the pistons 62 through first cylinder outlet 81, the fluid forcing valve means 87 open, and flows into an outlet chamber or cooling chamber 93 formed in rearhousing portion 24. Outlet chamber 93 is isolated from the inlet fluid in inlet chamber 86 by an annular floating barrel driving ring and seal 95 engaging barrel 70 and an annular cylinder barrel driving piston, indicated generally at 146 which will be described in detail later herein.

Outlet chamber 93 completely surrounds cylinder barrel assembly 70 and forms a passage means for the outlet flow of fluid through a check valve 220 disposed in a housing outlet port 222 formed in the rear housing portion 24.

As best seen in FIG. 1, as a piston 62 advances during the compression stroke, outlet port 81 is closed by the outer wall of cylinder 78 and flexible reed valve 87 will close.

It is important to pointout that the flow through outlet port 81 depends upon the position of barrel 70,

hence port 81, relative to the stroke of piston 62. Further, since the fluid from outlet port 81 flows completely around barrel 70, a very effective cooling flow is developed.

When piston 62 advances to the position where outlet port 81 is closed, the pressurized fluid is then discharged through the second cylinder outlet port 83 through longitudinal passages 10?. in reaction plugs 90, a plurality of small radially extending passages 103, annular recesses 104, an annular manifold, not illustrated, barrel outlet port 122, radial passage 112, an outlet member 110, passage 114, and a second housing outlet port 116 to the load.

Hollow outlet member 110 includes the central passage 112 that communicates with high pressure discharge passage 114 that in turn leads to housing outlet port 116.

As seen in FIG. 1, outlet member 110 also includes a foot portion provided with a surface 118 that is in slideable sealed engagement with a longitudinally extending surface 120 formed in the outer wall .of cylinder barrel 70. l

It will be noted from FIG. 1 that when cylinder barrel is axially shifted relative to the hotlsingjmeans 20 an outlet port 122 formed in the cylinder barrel always remains in communication with central passage 112 in outlet member notwithstanding axial movement of cylinder barrel 70. l

With continued reference to FIG. 1, pressure biased outlet member 110 includes a piston surface 124 that causes the pressurized hydraulic fluid in passage 112 to bias the surface 11.8 on outlet member 110 downwardly into sealed engagement with longitudinally extending surface on barrel 70.

A spring 126 augments the-biasing force of the high pressure oil on piston surface 124 and also serves to retain surface 1 18 in sealed engagement with surface 120 at low pressures and at the outset of operation.

The outer peripheral surface of outlet member 110 is provided with an annular seal 128 and a threaded plug 129 is screwed into the hole forming passage 112 and includes an inner protrusion that forms aretainer for the end of spring 126.

Pressurized oil is also released to a variable displacement hydraulic control unit indicated generally at by a second pressure biased outlet member 110-A, FIG. 1 which is substantially identical to outlet member 110 previously described. It will be noted that outlet member 110-A includes a base surface 136 that is hydraulically biased into'sealed engagementwith a longitudinally extending surface 138 formed in the outer wall of cylinder barrel 70. Outlet member 110-A is biased downwardly against longitudinally extending surface 138 by a force exerted by a piston surface 124 subjected to pressurized oil in a passage 140.

Referring to FIG. 1, cylinder barrel 70 is constantly biased towards the front of the housing means by a control spring 142 which is interposed between a spider 143 and an annular shoulder 145 formed on cylinder barrel 70.

Cylinder barrel 70 is hydraulically shifted axially against the biasing force of control spring 142 by an annular cylinder barrel driving piston indicated generally at 146 in FIG. 1. Piston 146 is mounted in a cylindrical surface 148 and forms therewith control cylinder 150 for receiving pressurized oil in a manner later to be described. A small annular piston surface 152 of large diameter provides sufficient axial force with low control pressures to shift Cylinder barrel 70 against the force of control spring 142.

With continued reference to FIG. 1, piston 146 includes a rear end 155 in force transmitting engagement with floating barrel driving ring and seal 95, which in turn is in force transmitting engagement with an annular base portion 157 on cylinder barrel cartridge 70.

Referring particularly to FIG. 1, pressurized oil is delivered through control 135 to control cylinder 150 by means of passage 140 and 162 and the details of the function of and structure of this control apparatus is described in detail in my above US. Pat. No. 3,357,363.

When control cylinder 150 is pressurized, and cylinder barrel 70 is shifted upwardly the cylinder barrel is adapted to shift rapidly by the inclusionof a by-pass flow chamber relief apparatus. Such apparatus comprises a relief valve plate indicated generally at 300 which is formed from a thin sheet of spring steel or the like. As seen in FIG. 1, relief valve plate 300 engages an end surface 302 in cylinder barrel 70 and is retained in place by annular floating barrel driving ring and seal It should be further pointed out that relief valve plate 300 is accurately located and retained in place against radial shifting by the side walls of pumping pistons 62 which confront inwardly facing arcuate surfaces formed on relief valve plate 300. Such relief valve plate is shown in detail in my above mentioned US. Pat. No. 3,357,363.

With continued reference to FIG. 1, the pumping pistons 62 are returned and biased against cam means 44 by a single centrally disposed return rod 194 which includes an arcuate socket 196. A ball 198 fits into a socket 200 formed in a piston return yoke 202. Yoke 202 includes a plurality of circumferentially spaced holes 204 that fit around neck portions 206 and which are large enough to permit free oscillation of neck portions 206 of the piston means 62.

The periphery of yoke 202 further includes a plurality of piston shoe mounting recesses that are shaped to form snugly fitting sockets for the tops of the piston shoe means.

Piston return yoke 202 applies force tothe rear sides of the ball-shaped piston ends and in turn receives force from piston return rod 194 via the pivot joint formed by ball 198 and socket 200.

Pulse filtering apparatus 310 is shown and described in detail in my above mentioned US. Pat. No. 3,357,363.

It is important to point out that the present invention has been described only by way of illustration, with respect to variable displacement operation and a cooling circuit but is not limited to such an application.

Reference is next made to FIGS. 2 and 3 which illustrate in isolated relationship a relief clutch apparatus constructed in accordance with the present invention, which apparatus functions to automatically relieve the pistons of the driving force of the cam upon the occurrence of any abnormal force imposed on any one of the pistons. An example would be a force imposed by binding action between a piston and cylinder wall due to the presence of contaminants in the hydraulic fluid.

Referring particularly to FIGS. 2 and 3, the former illustrates the relief clutch mechanism in its normal operational position whereas FIG. 3 illustrates the same mechanism in its released position wherein the pistons are isolated from the driving force of cam means 44.

As seen in FIG. 3 the previously described piston return rod 194 includes a cylindrical cavity 422 in which is mounted an upper latch member 416 and a lower latch member 416-A Such latch members are normally maintained in their spaced operational position of FIG. 2 by a jaw actuating pin indicated generally at 408. As seen in FIG. 2, a left shoulder 410 of the pin is normally disposed between left spaced ends of jaw members 416 and 416-A and the right end of pin 408 includes an enlarged head portion 412 which is normally disposed between the right ends of the upper and lower latch members. Hence it will be understood that during normal operation the latch members form spring engaging shoulders 418 for maintaining compression of the previously described cam biasing spring 210.

It should further be mentioned that the right end of jaw actuating pin 408 is engaged by a normal position biasing spring 422 which is disposed in space 420.

Also it will be noted that each of the latch members 416 and 416-A is provided with a guide or positioning flange 223 of cylindrical shape which conforms with the shape of the inner surface of piston return rod 194 so as to align the latch assembly in the cavity.

The latch members 416 and 416-A are released when the left end of jaw actuating pin 408 is caused to engage adjustable pin engaging member 402. In such instance the shoulders 410 and 412 of jaw actuating pin 408 are longitudinally arrested, when cam 202 moves from the position of FIG. 2 to the position of FIG. 3 with the result that latch members 416 and 416-A move to the left beyond such shoulders 410 and 412 and are freed to collapse or move inwardly to the position of FIG. 3. When this occurs, spring releasing shoulders 418 move inwardly thereby releasing cam biasing spring 210 from the compressed position of FIG. 2 to the extended released position of FIG. 3. As a result, the piston return force normally imposed by cam biasing spring 210 is released from its biasing effect on piston return rod 194. Hence it will be understood that the piston feet 56 will no longer follow cam 202 and hence reciprocation of piston 62 is immediately terminated.

With further reference to the embodiments of FIGS. 2 and 3, the above described pin engaging member 402 includes a threaded junction 404 with a longitudinal bore through spring retainer 216. Hence pin engaging member 402 can be longitudinally adjusted upon inserting a screwdriver into its slotted outer end.

Reference is next made to FIG. 4 which is a force diagram provided for the purpose of aiding in the description of the operation of the present relief clutch apparatus. Assuming that the pump is operating and the pistons 62 are reciprocating, in the event that one of the pistons should galled due to contamination, such piston 62 will be arrested by the occurrence of obstructive material or distortion at the area of contact with cylinder wall 78. Such obstructive material or distortion will cause the cylinder wall to impose a frictional force F j on the obstructed piston 62 and arrest its movement. When this occurs piston return yoke 202 and the piston foot 56 of the galled piston will be restrained by the obstruction from following the cam means 44. As the cam means 44 progresses with its rotation through a fraction of a cycle, the portion of the return yoke 202 at the un stuck pistons will be driven inwardly to the released position of FIG. 3. As piston return yoke 202 does this it willof course shift piston return rod 194 to the left causing jaw actuating pin 408 to engage stationary pin engaging member 402. This causes latch members 416 and 4l6-A to collapse inwardly, as previously described, thereby releasing piston return spring 210 from its biasing effect on piston return yoke 202. Pistons 62 can no longer follow cam 44 and their reciprocation is therefore terminated.

With continued reference to FIG. 4, when one of the pistons 62 is resisted by frictional force F at its cylinder wall contact 78 the force F of the cam is imposed on piston return yoke 202 with an effective lever arm length of L, with the piston opposite to the arrested one being moved inwardly a distance Ax. At the same time piston return yoke 202 will apply a force on piston return rod 194 with an effective lever length L Hence it will be understood that a bending moment is imposed on return yoke 202 by cam 44 due to the arrested piston 62 which moment functions to overcome the force F, of cam biasing spring 210 and thereby move piston return rod 194 inwardly. Also it will be seen from FIG. 4 that the force effect on piston return rod 194 is multiplied bya factor of 2 due to the different lever arm lengths L and L Reference is next made to a modified embodiment of the relief clutch apparatus illustrated in FIG. 5. Here the elements of the clutch apparatus which are disposed within the cylinder barrel 70 are identical to those previously described with respect to the embodiment of FIGS. 2 and 3. However, the embodiment of FIG. 5 differs in that the relief clutch apparatus, including the force releasing latch elements 416 and 416-A, can be actuated and released from an external pressure signal.

The structure of the embodiment of FIG. 5 comprises a modified pump housing 24-A which includes a rearwardly extending fluid actuated cylinder 430 that slideably mounts a fluid actuated piston 436 provided with an O-ring seal 450. Piston 436, when extended from left to right, functions to shift a fluid actuated pin 442 to the right with respect to spring retainer 2l6-A in which such pin is slideably mounted. Upon shifting of pin 442 towards the right, as viewed in FIG. 5, such pin is caused to engage jaw actuating pin 408 which is in turn shifted to the right thereby causing the inward col lapse of upper and lower latch members 416 and 416- With continued reference to FIG. 5, hydraulically actuated pin 442 and piston 436 are biased towards normal position by a return spring 440 with such piston and pin being extendable against the spring upon the application of external fluid pressure via a conduit 438. Hence it will be understood that the release clutch apparatus of FIG. 5 can readily be actuated by a rise in system pressure merely by connecting conduit 438 with any selected portion of the hydraulic circuit. It will be understood that the embodiment of FIG. 5 can be made responsive to any external pressure, as well as being manually actuatable merely by connecting conduit 438 to a manually actuated pressure source.

As still another embodiment of the present invention the relief clutch apparatus can be caused to react to the occurence of excessive temperature within the pumping mechanism. This can readily be done by merely arranging the embodiment of FIGS. 2 and 3 such that the jaw actuating pin 410 consists of an alloy which melts at a precise preselected temperature, for example 250 Fahrenheit. Hence, upon the occurence of excessive heat build-up within the pumping apparatus the special jaw actuating pin, meltable at low temperature, will upon melting permit latch members 416 and 4l6-A to collapse inwardly, release the biasing effect of cam biasing spring 210, and thereby immediately arrest reciprocation of pistons 62 within their respective cylinders 78.

It should be mentioned that operation of the various above described relief clutch apparatus is substantially instantaneous, it having been determined by test operations that reciprocation of the pumping pistons 62 is terminated within a fraction of a single rotation of cam means 44. It will therefore be understood that damage to the pumping mechanism can most effectively be prevented.

While the forms of embodiments of the present invention as herein disclosed constitute preferred forms, it is to be understood that other forms might be adopted.

What is claimed is:

1. A hydraulic pump comprising, in combination, housing means, a plurality of cylinders in said housing means; a plurality of pistons disposed in said cylinders; drive shaft means including a cam engaging said pistons; a piston return yoke engaging said pistons; resilient means biasing said yoke against said pistons; and relief clutch means for modifying the biasing effect of said resilient means on said yoke to relieve the pistons of the driving force of the cam.

2. A hydraulic pump comprising, in combination, housing means, a plurality of cylinders in said housing means; a plurality of pistons disposed in said cylinders; drive shaft means including a cam engaging said pistons; a piston return yoke engaging said pistons; a piston return rod slideably mounted in said housing means and engaging said piston return yoke; a piston return spring between said piston return rod and said housing means for biasing said piston return rod against said yoke; and relief clutch means for disengaging said piston return spring from said piston return rod and thereby relieve the pistons of the driving force of the cam.

3. A hydraulic pump comprising, in combination, housing means, a plurality of cylinders in said housing means; a plurality of pistons disposed in said cylinders; drive shaft means including a cam engaging said pistons; a piston return yoke engaging said pistons; A piston return rod slideably mounted in said housing means and including a first end engaging said yoke and a second end provided with a bore; relief clutch jaw means disposed in said bore and laterally movable between a spring retaining position and a spring releasing position; a piston return spring normally disposed in compressed relationship between said relief clutch jaw means and said housing means; and actuating means for said relief clutch jaw means.

4. The apparatus defined in claim 3 wherein said actuating means comprises an actuating pin disposed in said jaw means for normally holding said jaw means in said spring retaining position, said actuating pin being adapted to release said jaw means to said spring releasing position.

5. The apparatus defined in claim 3 wherein said actuating means comprises a jaw actuating pin slideably mounted in said relief clutch jaw means between jaw retaining and jaw releasing positions; and a pin engaging member mounted on said housing means in confronting relationship with said actuating pin whereby engagement of said actuating pin with said pin engaging member shifts said pin and thereby releases said jaw means to said spring releasing position.

6. The apparatus defined in claim 3 wherein said actuatingmeans comprises a jaw actuating pin slideably mounted in said relief clutch jaw means between jaw pin releases said jaw means to said spring releasing position.

7. The apparatus defined in claim 3 wherein said actuating means comprises a jaw actuating pin slideably mounted in said relief clutch jaw means between jaw retaining and jaw releasing positions; and a pin engaging member mounted for longitudinal movement on said housing means between a released position and a pin engaging position, said pin engaging member including an outer end extending to the exterior of said housing whereby force exerted on said outer end shifts said pin engaging member into engagement with said jaw actuating pin whereby said pin is shifted to release said jaw means to said spring releasing position.

8. The apparatus defined in claim 7 that includes a pressure responsive sensing means .operatively associated with said outer end of said pin engaging member. l 

1. A hydraulic pump comprising, in combination, housing means, a plurality of cylinders in said housing means; a plurality of pistons disposed in said cylinders; drive shaft means including a cam engaging said pistons; a piston return yoke engaging said pistons; resilient means biasing said yoke against said pistons; and relief clutch means for modifying the biasing effect of said resilient means on said yoke to relieve the pistons of the driving force of the cam.
 2. A hydraulic pump comprising, in combination, housing means, a plurality of cylinders in said housing means; a plurality of pistons disposed in said cylinders; drive shaft means including a cam engaging said pistons; a piston return yoke engaging said pistons; a piston return rod slideably mounted in said housing means and engaging said piston return yoke; a piston return spring between said piston return rod and said housing means for biasing said piston return rod against said yoke; and relief clutch means for disengaging said piston return spring from said piston return rod and thereby relieve the pistons of the driving force of the cam.
 3. A hydraulic pump comprising, in combination, housing means, a plurality of cylinders in said housing means; a plurality of pistons disposed in said cylinders; drive shaft means including a cam engaging said pistons; a piston return yoke engaging said pistons; A piston return rod slideably mounted in said housing means and including a first end engaging said yoke and a second end provided with a bore; relief clutch jaw means disposed in said bore and laterally movable between a spring retaining position and a spring releasing position; a piston return spring normally disposed in compressed relationship between said relief clutch jaw means and said housing means; and actuating means for said relief clutch jaw means.
 4. The apparatus defined in claim 3 wherein said actuating means comprises an actuating pin disposed in said jaw means for normally holding said jaw means in said spring retaining position, said actuating pin being adapted to release said jaw means to said spring releasing position.
 5. The apparatus defined in claim 3 wherein said actuating means comprises a jaw actuating pin slideably mounted in said rElief clutch jaw means between jaw retaining and jaw releasing positions; and a pin engaging member mounted on said housing means in confronting relationship with said actuating pin whereby engagement of said actuating pin with said pin engaging member shifts said pin and thereby releases said jaw means to said spring releasing position.
 6. The apparatus defined in claim 3 wherein said actuating means comprises a jaw actuating pin slideably mounted in said relief clutch jaw means between jaw retaining and jaw releasing positions; and a pin engaging member mounted on said housing means in confronting relationship with said actuating pin, said piston return rod, relief clutch jaw means, and actuating pin being longitudinally shiftable by the occurrence of a bending moment imposed on said piston return yoke as a result of excessive frictional drag occurring between one of said pistons and cylinders, said longitudinal shifting serving to cause said jaw actuating pin to engage said pin engaging member whereby said actuating pin releases said jaw means to said spring releasing position.
 7. The apparatus defined in claim 3 wherein said actuating means comprises a jaw actuating pin slideably mounted in said relief clutch jaw means between jaw retaining and jaw releasing positions; and a pin engaging member mounted for longitudinal movement on said housing means between a released position and a pin engaging position, said pin engaging member including an outer end extending to the exterior of said housing whereby force exerted on said outer end shifts said pin engaging member into engagement with said jaw actuating pin whereby said pin is shifted to release said jaw means to said spring releasing position.
 8. The apparatus defined in claim 7 that includes a pressure responsive sensing means operatively associated with said outer end of said pin engaging member. 